The present invention relates to a stabilizer composition which is useful for stabilizing a thermoplastic polymer against discoloration, to a polymer composition containing a thermoplastic polymer and the stabilizer composition and to shaped articles produced from the polymer composition.
It is generally known in the art that most of the known thermoplastic polymers are affected to a certain extent when they are exposed to heat, humidity and/or UV radiation. The deterioration of the polymers by heat and/or UV radiation often results in yellowing of the polymer and in degradation of its molar mass. Therefore, it is common to treat thermoplastic polymers with stabilizers. A large variety of stabilizers has been suggested in the prior art for the various thermoplastic polymers. For example it has been suggested to utilize certain organic phosphites, phosphonites or phosphines, optionally in combination with epoxides or polydialkylsiloxanes, for stabilizing polycarbonates against thermal or thermal-oxidative degradation. The stabilization of polycarbonates by means of phosphines is for example known from U.S. Pat. No. 4,092,288 and from U.S. Pat. No. 4,627,949. U.S. Pat. No. 4,627,949 discloses that shaped polycarbonate articles which have been stabilized by means of phosphines are useful in all cases where the shaped articles are exposed to high temperatures for a relatively long time, and in addition for all articles for which a high transparency is required, especially in the light field, for example for lamp covers or glazing with polycarbonate sheets.
Furthermore, it is known that polycarbonate hydrolyzes when it is contacted with boiling water. The hydrolysis of the polycarbonate is accelerated in the presence of thermostabilizers, such as phosphites or phosphonites. Epoxides or organic silicon compounds are known to stabilize polycarbonate against hydrolyric degradation.
It is also known that the influence of UV light, particularly in combination with oxygen and humidity, attacks polycarbonate by photooxidation. Benzotriazole and benzophenone systems as well as acrylic acid esters and malonic acid derivatives have been suggested as UV stabilizers.
U.S. Pat. No. 4,722,955 discloses carbonate polymer compositions which comprise a carbonate polymer, a heat stabilizing phosphorous compound, an epoxy compound containing internal acyclic ether linkages and optionally a hindered phenol. The phosphorous compound is an organo-phosphine, an organophospate or, preferably, an organo-phosphonite or an organo-phosphite. Although these carbonate polymer compositions have good optical properties and improved optical stability, as compared to other known carbonate polymer compositions, the color stability of these compositions at very high temperature, e.g. at 280.degree. C. or more, is still not as great as would be desirable.
Although a wide variety of stabilizers exist for the various types of thermoplastic polymers, it is still desirable to provide new stabilizers in order to meet the ever increasing quality requirements for thermoplastic polymers.